1. Field of the Invention
The present invention relates to temperature monitoring devices for monitoring the temperature distribution of a furnace or kiln.
2. Description of the Prior Art
Furnaces used in the semiconductor industry are operated to generate a nearly flat profile at around 1200.degree. C. or higher for lengths on the order of three feet or less. The furnaces used are usually of the tube type, having an overall length on the order of six feet. These furnaces are circular in cross-section and vary from two inches to eight inches in diameter. It is generally assumed that the temperature along any given diameter of the tube is the same as that at any other point along the same diameter. The temperature control for this type of furnace is designed to regulate the temperature along the axis of the tube. A flat profile for the furnace, therefore, means that the temperature at one point along the axis of the tube is the same as the temperature at all other points along the axis for points within the flat zone of the furnace.
The flat zone of the furnace is maintained by regulating the power to each of three heating zones within the furnace. The temperature is measured at one point (usually in the center) of each heating zone by a thermocouple, and the feedback from this thermocouple regulates the power to the zone. The heating zones may typically be ten inches to 20 inches long, and the control thermocouples reside outside of the combustion tube whose axis is coincident with that of the furnace.
Profile monitoring in such furnaces is accomplished generally by installing three accurately calibrated thermocouples at various positions along the axis and inside the combustion tube. When all three of these thermocouples register the same temperature, the profile is assumed to be flat. The degree of flatness is determined by the amounts the thermocouple outputs vary. Usually a flat profile requires that the temperature at any point along the axis does not vary by more than .+-.1/2.degree. C. At times, larger variations can be tolerated, but present furnace designs attempt to meet a .+-.1/2.degree. C. requirement.
There are several disadvantages in this type of profiling technique. Only three points within a relatively long flat zone area are measured at any given time as an indication of the temperature within that zone. The cost of thermocouples capable of accurately determining temperatures in the 1200.degree. C. range and surviving an oxidizing environment is relatively high, and even thermocouples constructed of the most suitable materials need to be recalibrated periodically because of drift in EMF output with time at this temperature.
Attempts have been made to improve on the above-described system by moving the three thermocouples within the flat zone. This is a time-consuming process because the specific heat of the thermocouples requires that the thermocouple be allowed time to reach equilibrium at each given point it is moved to. It has been suggested that more than three thermocouples be used to measure the temperature profile. This is a high cost alternative which is tied directly to the cost of the thermocouple materials. The present invention discloses a thermocouple probe that has improved degradation resistance and has none of the shortcomings of prior art temperature profile measuring techniques.